As a revenge killer, it won't need to OHKO anything so HP Fight is by far the better choice. As a sweeper, then it's just an accuracy vs power preference.
So really, it comes down to what you want Gengar to do. Sweep by itself (in which case, use Life Orb or Specs), or revenge kill (Scarf and HP Fight).
As for needing something to deal with dragons, STAB Shadow Ball will take care of revenge killing most, and Desitny Bond/Explosion can be used when all other options won't guarantee a KO. HP Ice does have the added benefit of hitting Zapdos, but it can take quite a beating from Gengar when it doesn't have Life Orb or Specs.
Oh, one other note on your now Scarf Gengar set: Most Scarf Gengars are Timid and used as starters. If your guy is coming out in the mid to late game, use a Modest nature instead. It gives it a bit more of a punch, and you won't have to worry about other Scarf Gengars.
My other concern is Salamence. Brick Break, as the sole physical attack, seems like a bit of a waste. Unless you're really worried about late game screens, I think any other physical attack would be better.
